1. Field of the Invention
The present invention relates to a focus state detecting optical system, for detecting the focus state of an objective lens by means of the light coming from an object and transmitted by said objective lens.
2. Related Background Art
There have already been made various proposals on an optical system, for detecting the focus state of a photographing lens, to be placed behind a primary image formed by said photographing lens thereby reforming said primary image as mutually similar two secondary images, wherein said focus state is determined from the positional aberration of said two images.
For example, the Japanese Laid-open Patent Application Nos. 59-75209 and 60-32012 propose to provide an aspherical condenser lens in the vicinity of or behind the plane of primary image of the photographing lens, thereby realizing uniform distribution of light quantity reaching the sensors and correcting the distortion aberration.
Also the U.S. Pat. No. 4,699,493 proposed to improve the symmetric nature of the secondary image plane by deviating each central axis of a pair of diaphragms serving as the pupil dividing means from each optical axis of a pair of imaging lenses for forming the secondary images on the sensors, or by providing a face of prism function on said imaging lens.
The two secondary images, formed at the position of sensors for detecting the focus state, have to be of symmetrical properties, with respect to a point on each sensor, corresponding to the central point of an effective area (hereinafter called distance measuring area) on the primary image plane of the photographic lens.
However, the technologies disclosed in said Japanese Laid-open Patent Application Nos. 59-75209 and 60-32012 cannot be considered to provide sufficient correction on the symmetric and imaging properties of the secondary images formed on said sensors.
In order that the focus state can be detected with a constant accuracy regardless of the position of the images on said secondary image plane, the symmetric properties alone of the secondary images formed on said sensors are not sufficient, but there are required uniform and satisfactory imaging properties on said secondary images.
In order to attain such imaging properties, it is necessary to correct the curvature of image plane of the meridional image plane (hereinafter called a curvature of image plane) along the direction of arrangement of the sensors.
However, in the technology disclosed in the U.S. Pat. No. 4,699,493, there still remains a curvature of the image plane, so that the unevenness of the image cannot be sufficiently corrected. More specifically, since the central portion and the peripheral portion of the distance measuring area show considerably different imaging properties on the secondary image plane, the accuracy of distance measurement with the sensors cannot be uniform.
In order to resolve such drawbacks, the Japanese Laid-open Patent Application No. 62-25715 proposes to provide an auxiliary lens, coaxial with the condenser lens, at the object side of the dividing optical system, but the increase in the number of optical members will result in additional difficulties such as the additional adjustment required in the manufacture or the increased cost.
On the other hand, if the distance measuring area is enlarged by increasing the image (reducing) magnification of the secondary images, the symmetric and imaging properties of said images become more critical.